In recent years, from the standpoint of reducing the burden on patients, improving diagnostic accuracy, etc., endoscopes have been reduced in size and improved in terms of image quality. Therefore, compact image-acquisition elements having high pixel numbers (for example, CODs and CMOSs) have been developed for use in endoscopes, and the pixel pitches thereof have been reduced year by year. As the pixel pitches are reduced, the endoscope objective optical systems are also required to be reduced in size, while achieving a wide angle of view and satisfying the required optical performance, such as aberration correction.
In general, when the pixel pitch of an image-acquisition element is reduced, the Fno needs to be reduced to make the objective optical system bright, otherwise a high-resolution image cannot be obtained. If the objective optical system is made bright, the depth of focus becomes small. Furthermore, in an objective optical system, after positioning, when the focus position is shifted due to, for example, displacement occurring when an adhesive is cured, the influence of a focus-position adjusting device, or the like.
Compact endoscope objective optical systems that can achieve high image quality are disclosed in, for example, 1 and 2. Specifically, PTL 1 discloses an endoscope objective optical system that has been reduced in diameter, has been made bright by reducing the Fno, has good various aberrations characteristics, and has been made less susceptible to one-side blur without increasing the power of the first lens. Furthermore, PTL 2 discloses an endoscope objective optical system that has good various aberrations characteristics.